A ball screw selection that reaches the RFQ stage without an axial-load calculation, a buckling check and a DN value is a quote, not a spec: engineers lock those numbers first, then match the screw to the duty cycle [S3].
Commercial options now span 6 mm to 80 mm shaft diameter in steel, with ground, rolled and thread-whirled variants competing on accuracy versus cost [S1]. Market offerings extend from sub-USD 10 wholesale units to precision ground assemblies in the USD 200 range, with DIN 69051 Form B accuracy classes appearing on Chinese export lines [S4].
Inputs the catalogue cannot give you
THK lists the irreducible input set as: transferred mass m (kg), guide method (sliding vs rolling), guide friction coefficient μ, guide resistance f (N), external axial load F (N), service life Lh (h), stroke ℓs (mm), maximum speed Vmax (m/s), and the three time slices — acceleration t1, constant speed t2, deceleration t3 — that drive the equivalent load [S3].
Without the cycle shape, a rated dynamic load Ca is decorative. MISUMI's worked example converts a 50 kg mass on rolling guides (μ = 0.02) into a constant-velocity axial load of about 10 N and a much larger acceleration-phase load, then uses the higher figure as Pb in the life equation [S7]. The lesson is mechanical: the worst phase wins, not the average.
Buckling, compressive load and the shaft equation
THK treats buckling of the screw shaft as a hard ceiling: the screw must not buckle under the maximum compressive axial load, with a free-shaft Euler boundary typically governing vertical and long-stroke horizontals [S2].
Two derived numbers flow from this check. The first is the permissible compressive load, used to set the static safety factor (commonly 2.0–4.0 for machine-tool axes). The second is the critical speed Nc, which couples with DN value (the product of shaft diameter in mm and rotational speed in rpm) to set the upper mechanical speed bound [S2]. Ground screws with recirculation usually carry lower DN ceilings than whirled or large-lead rolled screws because recirculation geometry caps ball velocity at the return path.
DN value, lead accuracy and the three manufacturing tiers
NTN Europe markets the BSC family in three manufacturing paths — rolled, whirled and ground — across the 6–80 mm diameter range, with each path occupying a distinct price-accuracy pocket [S1]. Rolled is the cost baseline, thread-whirled sits in the middle on lead accuracy and surface finish, and ground sets the accuracy ceiling.
Accuracy is graded by lead error per 300 mm, with DIN 69051 Form B (and tighter Form 0/1) cited on Chinese export SKUs such as SFNU05020-4 [S4]. Preload class — slight (Z0), light (Z1), medium (Z2) or heavy (Z3) — is selected from the ratio of external load to dynamic load, not from the catalogue page; overspecifying preload burns efficiency and heat, underspecifying it shows up as lost motion on reversal.
Equivalent load, life and the cubic curve
Ball screw life is rated in revolutions L10, then converted to hours by the mean speed. The conversion from a variable cycle to a single equivalent load uses the cubic-power weighted mean of the load histogram — a step a manual selection often skips and a selection tool such as THK's e-LINE, MISUMI's Technical Calculation Software Ver.2.0, or Kuroda's actuator selection tool handles directly [S7][S8].
Kuroda's tool ties a target service life in hours to the result panel alongside stroke, speed and guide-rail length, which is the practical way to keep "50,000 h" or "20,000 km" promises honest [S8]. A short, fast cycle on a lightly loaded screw can still kill the nut in a fraction of the rated life if the equivalent-load conversion is ignored.
Comparison frame: rolled vs whirled vs ground
Against four decision criteria, the manufacturing paths line up as follows, drawn from NTN Europe's tiering and the DIN 69051 accuracy options visible on export SKUs [S1][S4]:
• Cost: rolled lowest, whirled mid, ground highest — the gap is often 3–8× per metre at matched diameter and lead.
• Lead accuracy per 300 mm: rolled typically ≥ 50 µm, whirled roughly 20–35 µm, ground sub-10 µm in DIN 69051 Form 0/1.
• DN ceiling and speed: rolled and whirled handle higher DN in many catalogues, but ground's smoother raceway geometry gives lower heat rise at sustained dmn (DN × lead) for machine-tool spindles.
• Typical fit: rolled for general automation, packaging and 3D printer axes; whirled for mid-accuracy CNC and semiconductor handlers; ground for machine tools, EDM and metrology stages.
Lubrication, seals and the failure modes nobody budgets
Three failure modes dominate the field. First, contamination ingress — chip swarf, coolant and dust — which the seal kit (UU, WW or end-cap with felt) has to match, not the catalogue default. Second, lubricant starvation on long-stroke verticals, where the ball track runs dry above the oil reservoir and a grease plug or pumped-oil circuit becomes mandatory. [S1]
Third, fatigue spalling on the loaded zone of the nut, the natural end-of-life mode that the L10 calc is meant to predict. A high-precision ground screw from a Chinese export line such as the SFNU05020-4 with DIN 69051 Form B accuracy still needs a lubricant schedule and a contamination plan, or the accuracy class is paid for and not used [S4]. Miniature ball screws in instruments follow the same logic, just with smaller reservoirs and tighter oil-metering [S5].
Standards, audit trail and the spec envelope
Engineers should pin the screw to a recognised accuracy norm — DIN 69051 for European machine tools, JIS B 1192 for Japanese programmes — and to a preload class before the supplier conversation [S4]. The model code then needs to carry the diameter, lead, ball circle diameter, nut length, preload class, seal type and lubrication grade; a code stripped of these tokens cannot be re-ordered consistently.
For related drive-train context, the linear guide selection frame is the natural upstream gate — the guide method (sliding vs rolling) and the friction coefficient feed straight into the equivalent axial load on the screw [S3]. On the motor side, the variable speed drive vs planetary reducer decision frame is what sets the maximum permissible speed Vmax that the screw must accept without breaching its DN ceiling.
Quick selection gate before RFQ
Lock these eight items before a model code leaves the desk: external axial load F (N), mean and peak speed, stroke ℓs, transferred mass, guide friction μ, target life Lh, accuracy class, and lubrication/seal class. Inputs not on the list (brand, colour, end-journal drawing) are the supplier's job, not the specifier's. [S2]
Two signals to track through 2026: a wider spread of DIN 69051 Form B ground screws under USD 200 per metre on Chinese export channels, and tighter integration of selection software with supplier part numbers (THK, MISUMI, Kuroda all publish tool outputs that bind calculation inputs to a model code) [S4][S7][S8]. If your supplier cannot map your inputs to a Ca figure and an L10 hour count, the selection has not happened yet.
For component-level specifications, see ball screw, ball bearing, and ball spline.